One of the long recognized problems associated with molding articles is the adhesion of the molded article to the mold upon completion of the form determining process. Adhesion between the molded article and mold decreases the efficiency of manufacturing molded articles by requiring more force and time to remove the molded article from the mold. Often the molded article is so tenaciously adhered that separation of the article from the mold damages the surface of the molded product.
While adhesion is not completely understood, at least two types of adhesion can be identified which account for the adhesion of molded articles to molds. First, the process of molding intrinsically produces an intimate physical fit between the molded article and mold which requires some additional force to overcome. The second type of adhesion occurs when chemically curing the molding material is part of the molding process. Quite often the chemicals involved in cure mechanisms are aggressive adhesives, for instance, in molding polyurethane isocyanates are used as part of the resin forming process. These aggressive adhesives interact with the surface of the mold to form a chemical bond between the mold and the molded article.
Both types of adhesion can be minimized. Where adhesion is mainly attributable to the intimate physical contact between the molded article and mold, a fluid or flexible material disposed between the mold and molded article allows easier release of the article from the mold. Where chemical adhesion is present, the provision of a chemically inert mold surface decreases the chemical adhesion of the molded material to the mold surface.
Cured and uncured organosilicon materials have been used for a number of years as mold release agents. Uncured silicone materials have been used extensively in molding applications, but in the polyurethane foam molding applications such materials are undesirable because they transfer into the polyurethane reagents, and defoam the polyurethane foam.
Cured silicone compositions do not transfer into the molding materials, and are thus very useful in foam molding applications. However, many cured compositions do not adhere to the mold sufficiently to allow the mold to be reused without recoating, or are not sufficiently hard to resist the abrasions which occur during the molding process. Often materials which provide sufficient abrasion resistance have been found to cure so slowly that their application is impractical given the constraints of typical manufacturing techniques. Other compositions have been found to interact with the molding composition in such a way that the mold release characteristics of the cured silicone composition rapidly deteriorate.
U.S. Pat. No. 2,639,213 issued to Barth discloses the use of liquefied gas in silicone based oil as a mold release agent. Like most uncured systems, this release agent will defoam molded foams as will the mold release agents containing silicone described in U.S. Pat. No. 2,811,408 issued to Braley which describes a composition comprising a polysiloxane resin, and a polysiloxane oil which is useful in molding organic and organosilicon plastic articles.
U.S. Pat. No. 3,552,202 issued to Wada, et al. discloses a composition comprising .alpha..omega. hydroxyl end-blocked polydimethylsiloxane, polymethylhydrogensiloxane and polymethylphenylsiloxane. The mixture is claimed to form a film upon application to a surface which provides nonsticky surfaces to sticky materials.
U.S. Pat. No. 3,492,394 issued to Heine illustrates the organic solvent solutions of perfluoroalkyltrialkoxy silane to form durable mold release films. Heine speculates that the hydrolyzable groups react with the adventitious water and the mold surface to form a cured film.
U.S. Pat. No. 3,624,190 issued to Cekada, et al., discloses the use of polydimethylsiloxane fluid in conjunction with "typical release agents" to provide effective release coatings for polyurethane foam mold applications. The polydimethylsiloxane fluid is not cured in this composition, and its release into the molding reagent defoams certain polyurethane formulations.
Oppliger, Canadian Pat. No. 624,114, teaches that cured films of polysiloxane oils or gums act as release coatings in polyurethane foam molding applications. Oppliger does not teach that these films give multiple releases, and the examples and the compositions taught, and materials claimed in fact do not give multiple releases of molded polyurethane foam articles.
U.S. Pat. No. 3,634,764 issued to Brooks discloses a release coating composition comprised of methyl-terminated polydimethylsiloxane, partially methoxylated polymethylpropylsiloxane resin, and curing agent such as tetraalkyltitinates or metal salts of carboxylic acids. The polydimethylsiloxane is not cured in this coating and intermixes with the molded material. In polyurethane foam applications the uncured polydimethylsiloxane intermixes with the molding material, defoams the surface of the molded article, and forms an undesirable skin.
U.S. Pat. No. 4,244,912 issued to Battice teaches that a composition comprised of vinyl end-blocked polydimethylsiloxane, vinyl group containing polysiloxane resin, polymethylhydrogensiloxane cross-linking agent, and a platinum containing hydrosilation catalyst is useful as a multiple release coating. This coating gives multiple releases of molded polyurethane foam articles, and does not defoam the surface of such articles. The coating is difficult to recoat, however, because the cure system is inhibited by amines. Providing new cured coatings over spent coatings is difficult to insure in practice.
U.S. Pat. No. 4,534,928 issued to Martin teaches multiple release coatings for polyurethane foam molding applications comprised of predominantly hydroxyl terminated polydimethylsiloxanes; trialkoxysilane; metal alkoxides; and nonreactive organic solvent. This composition is applied to a mold surface, the solvent is driven off, and a cured film is formed which has multiple release properties. The coating defoams the surface of certain formulations of polyurethane foam molded articles.
U.S. Pat. No. 3,579,469 issued to Grenoble discloses a paper release coating comprised of a silanol terminated siloxane gum; a silane of the formula, HSiX.sub.3, where X is a hydrolyzable group; a metal salt of an organic acid; and a nonreactive organic solvent. This composition is not taught to be a mold release composition.
It is an object of this invention to provide an organosilicone coating which permits multiple releases of molded articles between successive applications of the coating. It is also an object of the invention to provide a mold coating which provides multiple releases of molded polyurethane foam articles. It is a further object of this invention to provide mold release coatings which do not defoam the surface of molded polyurethane foam articles and which may be readily recoated. Finally, it is an object of the invention to provide a mold coating with multiple release characteristics in molded polyurethane foam applications which cures to a nontacky state in less than ten minutes.